The invention relates to a knapsack or back pack with at lest one reinforcing element which is curved concavely, as seen from the outside, at least region-by-region in the longitudinal direction and/or in the transverse direction. Such element is received in pockets provided in a rear wall of the knapsack, and enables the rear wall to be deformed concavely in the longitudinal direction and/or in the transverse direction. A support net is stretched between a bottom region of the reinforcing element, e.g. by an elastic supporting strip extending transversely in the bottom region of the reinforcing element, and an upper region of the reinforcing element.
Such a back pack is known from EP-B-0 158 154 wherein there are disclosed two individual, essentially vertical reinforcing members which merge at the top and whose undersides are connected by another horizontal reinforcing member. The upper ends of the vertical reinforcing members are connected together by an additional horizontal connecting piece which projects sideways beyond the ends of the vertical reinforcing members. An elastic support, over which a support net extends, is arranged above the upper horizontal connecting piece. In this manner air ventilation in the back region is presumably achieved during use of the back pack. From the point of view of production and assembly, however, such arrangement is very expensive. In addition, the framework comprising vertical and horizontal reinforcing members and connecting pieces must be almost completely accommodated in a rear side pocket.